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1.
The isotopic composition of strontium of pore water and of authigenic minerals leached from the sediment of core 119K with hot aqua regia is similar to that of the brine in the Discovery deep and differs from that of normal seawater. The average ratio of strontium removed by acid leaching is 0.7077 ± 0.0007 (1σ) compared to a value of 0.70904 for the Red Sea. The detrital silicate fraction exhibits an approximate inverse correlation between ratios and strontium concentrations which provides tentative support for a model in which the detrital silicate fraction of deep-sea sediment is considered to be a mixture of terrigenous dust of sialic composition enriched in radiogenic 87Sr and of volcanogenic material of basaltic composition and low 87Sr abundance. The ratios of the shells of foraminifers and pteropods, expressed as δ 87Sr‰ relative to 0.70904 for seawater, decrease from at 90 cm to at 273 cm and remain constant at this value to a depth of 450 cm. The lowering of the δ 87Sr values is attributed both to the presence of aragonite overgrowths on pteropod shells and to possible isotope exchange with strontium in the connate fluid. 相似文献
2.
The South Mountain batholith of southwestern Nova Scotia is a large, peraluminous, granodiorite-granite complex which intrudes mainly greenschist facies metasediments of the Cambro-Ordovician Meguma Group. Using Rb-Sr isochrons constructed from whole rocks and mineral separates, the present study shows a variation in age and initial ratios of the intrusive phases of the batholith as follows: biotite granodiorite (371.8 ± 2.2 Ma, ( ranges from 0.7076 ± 0.0003 to 0.7090 ± 0.0003, with the average = 0.7081); adamellite (364.3 ± 1.3 Ma, ); porphyry (361.2 ± 1.4 Ma, ); using λ87Rb = 1.42 × 10?11yr?1.A suite of Meguma country rock samples showed a variation of at the time of intrusion of the batholith. A number of xenoliths of this material occurring in the marginal granodiorite had partially equilibrated isotopically with the granodiorite at a higher ratio than elsewhere in the granodiorites. This evidence demonstrates that isotopic (and probably some accompanying bulk chemical) contamination by the Meguma rocks has been an important factor in determining the ultimate chemical composition and mineralogy of the South Mountain batholith.The of the early granodiorites indicates that the parent magma of the South Mountain batholith was derived from a source unlike the Meguma Group. The precise nature of the source region cannot be determined by Rb-Sr work unless the degree of contamination with Megumalike material is known. 相似文献
3.
Martin Kralik 《Geochimica et cosmochimica acta》1984,48(3):527-533
Size fractions <5 μm of Fithian illite were treated with 1 N HCl, H+- from cation-exchange resin, and 5% NH4-EDTA-solution, and analyzed for their Rb-Sr isotopic composition. In comparison to the effects of HCl, the ion-exchange resin leached the same amount of Sr, but significantly less Rb and radiogenic 87Sr from poorly-crystallized 1 M illite.An age of 284 ± 8 Ma (I.R. 0.7109 ± 6 is more or less in agreement with the Missourian (Stephanian) age of deposition. The agreement is fortuitous because of the presence of small amounts of detrital 2 M illite and kaolinite, thus the true age of diagenesis might be somewhat less. ratio measurements of Tithonian Solnhofen limestone demonstrate that, even though the silicate component constitutes less than 5% of the total, the majority of the radiogenic 87Sr is leached from “broken-bond” surfaces of the nearly pure kaolinite-residues during the HCl treatment. Ion-exchange and EDTA treatment can be useful in this situation to obtain a precise separation of environmental Sr (in carbonate) from radiogenic 87Sr (in silicates), and an adequate spread of values from argillaceous fine fractions in the construction of isochrons. 相似文献
4.
D.W. Davis J. Gray G.l. Gumming H. Baadsgaard 《Geochimica et cosmochimica acta》1977,41(12):1745-1749
The decay constant 87Rb has been redetermined by measuring the amount of radiogenic 87Sr produced over a period of 19 years, in 20 g samples of purified RbClO4, using isotope dilution techniques. The rubidium sample was spiked with 84Sr and the nanogram quantities of strontium separated by coprecipitation with Ba(NO3)2. Analyses were carried out on a 25cm, 90° sector mass spectrometer equipped with a Spiraltron electron multiplier. Measurement of three independent ratios permitted continuous monitoring of the ion beam fractionation. The average of nine determinations gives a value for the decay constant of . [.] 相似文献
5.
Ján Veizer William Compston Norbert Clauer Manfred Schidlowski 《Geochimica et cosmochimica acta》1983,47(2):295-302
Strontium isotopic measurements were made on Late Proterozoic carbonates from West African Craton. Comparison of samples with acceptable trace element patterns with coeval data from southern Africa and with the published Australian results suggests that the ratio of the Late Proterozoic sea water evolved in the following manner about 0.7075 at 1000 ± 50 Ma, 0.7056 to 0.7074 at 900 ± 50 Ma, 0.7068 to 0.7091(0.7106) at 800 ± 50 Ma, 0.7074 to 0.7077 at 700 ± 50 Ma, and 0.7076 to 0.7089(0.7096) at 600 ± 50 Ma ago. The variations are comparable in magnitude and frequency to those described previously for the Phanerozoic. Strontium isotopic values in the radiogenic part of this range suggest that the continental river flux of Sr into Late Proterozoic oceans was of comparable isotopic composition to its present day counterpart (～0.711). Consequently, the non-radiogenic value at ～900 ± 50 Ma ago signifies a large flux of “mantle” strontium into the ocean at this time. Because the present time resolution is only about 75 ± 25Ma, additional sampling as well as better stratigraphie resolution and more definite selection criteria are required for construction of a more detailed Late Proterozoic sea water curve. 相似文献
6.
The Rameka Gabbro, emplaced 367 Ma ago, experienced a well documented reheating on intrusion of the Separation Point Batholith 114 Ma ago. age spectrum analyses of hornblende from the Rameka Gabbro show diffusion gradients which provide information on the 40Ar boundary concentration during reheating.Three samples of hornblende exhibit age spectra that conform to a model of 40Ar loss by diffusion, implying a zero 40Ar boundary concentration during heating. The calculated 40Ar loss from these samples, together with a model of heat flow in the aureole, provide estimates of diffusion coefficients of 40Ar in Mg-rich hornblende which correspond to an activation energy, E, of ~60 kcal-mol?1 and a frequency factor. D0, of ~ 10?3 cm2-sec?1. When combined with laboratory diffusion results, these data yield a well defined diffusion law (E = 63.3 ± 1.7 kcal-mol?1, ).The age spectra of the eight other samples record steep gradients of excess 40Ar over the first few percent of gas release. Although this effect causes high apparent conventional K-Ar ages, the plateau segments of many sampes still record the crystallization age of 367 ± 5 Ma. These measurements show that the excess 40Ar phase developed locally in the intergranular regions of the gabbro, following intrusion of the batholith. on time scales that varied from 104 to 106years. The minimum average ratio of this component was found to be 1300 ± 400. The partial pressure of Ar was at least 10?2 bars in some places.A single age spectrum analysis of plagioclase reveals a ‘saddle-shaped” release pattern with a minimum at 140 Ma.In conjunction with theoretical diffusion models and a diffusion law, age spectrum analysis of hornblende that has experienced a post-crystallization heating can provide close estimates of the maximum temperature of the thermal event as well as both age of crystallization and reheating. 相似文献
7.
8.
Surendra P. Verma 《Chemical Geology》1984,46(1):37-53
Twelve samples of mid-Tertiary felsic volcanic rocks from Zacatecas and San Luis Potosí (both belonging to the Sierra Madre Occidental) and one sample of Lower Tertiary porphyritic andesite from Zacatecas are analyzed for , K, Rb, and Sr. Eight selected samples are also analyzed for . A linear regression of the present-day of the felsic volcanic rocks in Zacatecas gives an approximate date of 30 ± 8 Ma. The initial ratios are high and widely distributed (from 0.705 to 0.712 or higher) whereas the initial ratios are somewhat low and show a narrow range (0.5125–0.5127). The available isotopic and trace-element data are best explained in terms of a binary mixing model in which the magmas derived from a slightly depleted-mantle fractionate and mix with varying proportions of the overlying middle/upper continental crust and undergo further shallow-level fractional crystallization before eruption. This model is also compatible with the trace-element and Sr isotopic data published from other areas of the Sierra Madre Occidental for which a purely mantle origin has been proposed. 相似文献
9.
age spectrum analyses of three microcline separates from the Separation Point Batholith, northwest Nelson, New Zealand, which cooled slowly (~5°C-Ma?1) through the temperature zone of partial radiogenic 40Ar accumulation are characterized by a linear age increase over the first 65 percent of gas release with the lowest ages (~80 Ma) corresponding to the time that the samples cooled below about 100°C. The last 35 percent of 39Ar released from the microclines yields plateau ages (103,99 and 93 Ma) which reflect the different bulk mineral ages, and correspond to cooling temperatures between about 130 to 160°C. Theoretical calculations confirm the likelihood of diffusion gradients in feldspars cooling at rates ≤5°C-Ma?1. Diffusion parameters calculated from the 39Ar release yield an activation energy, E = 28.8 ± 1.9 kcal-mol?1, and a frequency factor/grain size parameter, . This Arrhenius relationship corresponds to a closure temperature of 132 ± 13°C which is very similar to the independently estimated temperature. From the observed diffusion compensation correlation, this implies an average diffusion half-width of about 3 μm, similar to the half-width of the perthite lamellae in the feldspars. The range in microcline K-Ar ages from the Separation Point Batholith is the result of relatively small temperature differences within the pluton during cooling. Comparison of the diffusion laws determined for microcline with those for anorthoclases and other homogeneous K-feldspars (E = 40 to 52 kcal-mol?1) reveals that Ar diffusion is more highly temperature dependent in the disordered structural state than in the ordered structural state. Previously published U-shaped age spectra are probably the result of the superimposition of excess 40Ar upon diffusion profiles of the kind described here. 相似文献
10.
Norbert Clauer Michel Hoffert Anne-Marie Karpoff 《Geochimica et cosmochimica acta》1982,46(12):2659-2664
Morphological, mineralogical, chemical and RbSr isotopic studies have been made on Fesmectites (nontronites) from southern Pacific red clays cored near the Marquisas Islands. These minerals have at the top of the core, an ratio of 0.70917 ± 0.00007, which indicates an authigenic origin in isotopic equilibrium with seawater. Weak leaching experiments with 1N HCl show that the nontronites also contain a volcanic component with a lower ratio which, combined with the morphology of the particles, suggests a transportation by bottom currents of clay formed elsewhere.During burial, the nontronites experience diagenetic modifications resulting in an increase in Fe, K and Rb contents, a concomitant decrease of Mg, Ca, Ti, Na and Sr, and a preferential migration of radiogenic 87Sr from the clays into the surrounding pore waters.The ratio of the Sr adsorbed on the outermost surfaces of the nontronites does not change with depth in the core, and is, therefore, independent of diagenetic influence, which is rather characterized by the ratios of the interstitial waters. The isotopic composition of both the adsorbed Sr and that of the pore fluids may yield useful information on the crystallization environment and subsequent history of deep sea red clays. 相似文献
11.
Rb/Sr geochronology on a folded greenstone-granitoid complex in the Agnew area, Western Australia, yields four distinct ages of igneous activity that conform with stratigraphic and intrusive relationships. They are (using , NBS 70A = 522 ppm Rb and 65.3 ppm Sr):
| Ma | 87Sr/86Sr initial (IR) | |
| (1) Differentiated gabbro-granophyre from a stratigraphically old (Kathleen Valley) greenstone sequence | ||
| (2) Voluminous tonalite, the Lawlers Tonalite | ||
| (3) A less voluminous leucogranite, and a large complex pegmatite cutting the Perseverance nickel orebody | ||
| (4) Aplitic leucotonalite (very minor volumes but widespread) |